1. Field of the Invention
The present invention relates to a technique of generating a display image by volume rendering.
2. Description of the Background Art
As a method of visualizing and displaying data indicative of a three-dimensional structure, a method called volume rendering is known. The volume rendering includes a method called ray casting.
The volume rendering is often used at the time of displaying a three-dimensional image, for example, as a medical-use image for a diagnosis in the medical field. For example, at the time of observing an affected area or the like while appropriately changing the direction of sight line, it is desired to switch a display image quickly so as not to apply stress on a user.
Various methods of increasing the speed of volume rendering have been consequently devised and proposed. Among them, volume rendering of a type is known in which each time the direction of sight line, that is, the projection plane is changed, first, a depth map indicative of a distribution of distances between the projection plane and a three-dimensional object is generated and sampling along each of sight lines is performed by using the generated depth map, so that volume data is visualized. It is known that an enormous amount of computation is necessary for generating a depth map indicative of the distance from a certain viewpoint to a three-dimensional object (for example, Japanese Patent Application Laid-Open No. 2000-105105).
The volume rendering using the depth map requires, however, some calculation time for regenerating a depth map each time the direction of sight line is changed. Consequently, it takes a long time to generate a display image and to switch the display image, and it is difficult to smoothly change a display image in accordance with the change of the projection plane, that is, the direction of sight line.